Voltammetry of immobilized microparticles

Cepria G, Garcia-Gareta E, Perez-Arantegui J (2005) Cadmium yellow detection and quantification by voltammetry of immobilized microparticles, Electroanalysis 17 1078-1084. 

Paraffin impregnated graphite electrode (PIGE) - electrode prepared from graphite rods by impregnation with melted paraffin under vacuum. These electrodes are not permeated by aqueous solutions and can be used for solution studies, as well as for immobilizing microparticles and microdroplets to study their electrochemistry. See also - carbon, - voltammetry of immobilized microparticles. 

Solid-state electrochemistry — is traditionally seen as that branch of electrochemistry which concerns (a) the -> charge transport processes in -> solid electrolytes, and (b) the electrode processes in - insertion electrodes (see also -> insertion electrochemistry). More recently, also any other electrochemical reactions of solid compounds and materials are considered as part of solid state electrochemistry. Solid-state electrochemical systems are of great importance in many fields of science and technology including -> batteries, - fuel cells, - electrocatalysis, -> photoelectrochemistry, - sensors, and - corrosion. There are many different experimental approaches and types of applicable compounds. In general, solid-state electrochemical studies can be performed on thin solid films (- surface-modified electrodes), microparticles (-> voltammetry of immobilized microparticles), and even with millimeter-size bulk materials immobilized on electrode surfaces or investigated with use of ultramicroelectrodes. The actual measurements can be performed with liquid or solid electrolytes. 

Many electrochemical conversions of solid compounds and materials, including for example the corrosion of metals and alloys or the electrochemical conversions of most battery materials, take place within a liquid electrolyte environment, with the classic approach to investigation comprising macro-sized electrodes. However, in order to obtain a comprehensive understanding ofthe mechanism ofthese solid-state electrochemical reactions, the simple technique of immobilizing small amounts of a solid compound/material on an inert electrode surface provides an easy, yet sometimes exclusive, access to their study. In this chapter is presented a survey of the recent developments of this approach, which is referred to as the voltammetry of immobilized microparticles (VIM). Attention is also focused on progress in the field of theoretical descriptions of solid-state electrochemical reactions. 

Since its introduction in 1989 [3,4], the voltammetry of (immobilized) microparticles (earlier termed abrasive stripping voltammetry) has attracted considerable attention and initiated a wide range of experimental and theory-based studies. As outlined in Section 6.1, the first decade of investigations has been vell reviewed [5-8], while a recent monograph [9] has included the details of any diverse applications up until late 2004. Consequently, whilst the following sections will focus on the period between 2005 and the present, any earlier contributions deemed relevant to an understanding of the current investigations will also be cited or discussed. 

Inorganic pigments and corrosion products of metals have also been studied intensively by using voltammetry of immobilized microparticles. In this way, it was possible to identify hematite as the main corrosion product of a modern steel sculpture in Valencia [64]. Iron (III) oxides and oxide-hydrates are characterized by reductive dissolution signals with very distinct peak potentials and signal shapes. 

Kovanda et al. [67] have employed not only the voltammetry of immobilized microparticles but also several solid-state analytical techniques when studying the thermal behavior of a layered Ni-Mn double hydroxide. The important advantage of these electrochemical experiments was that they proved the presence of substantial amounts of amorphous compounds. Moreover, distinct signals could be identified for the reductive dissolution of the different phases. 

Of particular interest was the way in which detailed information could be derived from voltammetric studies of clay minerals treated aerobically with iron (11) solutions. This caused the precipitation of a thin, active layer of iron (HI) oxi(hydroxides) which could later be used for the sorption of arsenate(V) ions [69]. By employing the voltammetry of immobilized microparticles, it was possible to distinguish different iron species, namely (i) ion-exchangeable, labile, or sorbed iron (HI) ions (ii) ferrihydrite or lepidocrocite and (iii) crystalline hematite or goethite. Cepria et al. subsequently employed the voltammetry of immobilized microparticles in the phase analysis of iron (III) oxides and oxi(hydroxides) in binary mixtures, as well as in cosmetic formulations [70]. 

The organic and organometallic complexes of transition metals are especially important in catalysis and photovoltaics, on the basis of their redox and electron-mediating properties. Whilst most complex compounds can be studied in (organic) solution-phase experiments, their solid-state electrochemistry (often in an aqueous electrolyte solution environment) is in general also easily accessible by attaching microcrystalline samples to the surface of electrodes. Quite often, the voltammetric characteristics of a complex in the solid state will differ remarkably from its characteristics monitored in solution. Consequently, chemical, physical or mechanistic data are each accessible via the voltammetry of immobilized microparticles. 

A selection of organic and organometallic compounds studied in recent years using the voltammetry of immobilized microparticles is listed in Tables 6.2 and 6.3 however, only selected contributions will be described briefly in the following sections. 

Domenech-Carbo et al. also showed the voltammetry of immobilized microparticles to be valuable in the unambiguous identification of dyes such as Curcuma and Safflower in microsamples of works of art and archaeological artifacts (see also Section 6.4.1) [140]. Here, the use of square-wave voltammetry in aqueous acetate or phosphate buffers led to the appearance of well-defined oxidation peaks ofthe dyes in the potential region of +0.65 to +0.25 V (versus Ag AgCl). 

Cepria and coworkers used the voltammetry of immobilized microparticles to detect and quantify the cadmium pigments (e.g., cadmium sulfide and cadmium sulfoselenide) used in artists paints, as well as in glasses, plastics, ceramics, and enamels [141]. For this, a simple, fast and reliable technique was developed that proved to be especially applicable for valuable art objects, as it was minimally invasive and required only nanogram quantities of material (see also Section 6.4.1). For quantification purposes, an abrasive stripping scan was used from + 0.3 V to —1.0 V, following a 10 s pre-treatment step at —1.5 V. The Cd oxidation peak was evaluated with respect to an internal AgCl calibration standard. 

Methods describing the electrochemical investigation of solid compounds and materials have significantly expanded to new possibilities over the last two decades. This chapter focuses on the use of a fairly new and straightforward method referred to as voltammetry of immobilized microparticles (VIM). Detailed reviews of the method are available elsewhere [la-c]. Beside applications in fundamental studies, this method proved to be especially valuable for the analysis of solid materials studied in archeometry [Id]. 

The Home Page of the Voltammetry of Immobilized Microparticles (http // www.vim.de.vu)... 

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